"Americans had been, if you will, swimming around in the idea of thanking God or family or country during the month of November without any ... national consistency or declared observance," Koehn said. "Lincoln, at the urging of Ms. Hale, decides to do this in 1863."

In the Thanksgiving proclamation, Koehn says there are echoes of another famous piece of writing that Lincoln would deliver the following month: the Gettysburg Address. They are particularly similar, she said, in the way he called upon Americans to look for the better parts of themselves.

"How do I look for the better angels of our nature?" Koehn said Lincoln considered. "If I'm going to do something that's a national act, like finishing the dome of the Capitol, which he did in the middle of the war, [or] by finishing the national railroad, which would go coast to coast, both huge symbols of national unity ... How do I do it and call us to what's good about us as a people?"